Refs #5637 - DHCP hostname commutative conflict fix

When I was reproducing a bug related to pull request #1430 I noticed that in my
case (Ruby 2.0) it's not the `self` object without hostname set, but always
`other` object.

This check should be commutative, I am not sure if Ruby optimizes the call or
this has changed somehow in the runtime. But since hostname of the new host is
always set (we have a validator for this), adding commutative property should
not work.

I believe this should fix a problem I have with OpenStack (Discovery) installer
(RHBZ#1152516).